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Saturday, October 29, 2011

Tube of the Month: The 6CG3

Hi!

The tube of the month of October is again a rectifier. When you read my blog, you know that I'm very fond of TV damper tubes as rectfiers and use them in almost every project. I already presented the 6AX4 which suitable for almost all my needs. This article is about a TV damper which can handle more current: The 6CG3.

The 6CG3 can handle a peak current of over 1A and a DC output current of 350mA. This means a pair of them in a full wave rectifier scheme with center tap can deliver up to 700mA of current when used with a choke input filter. The same for a full wave bridge with 4 of them. If you are conservative you can still easily draw half an ampere from a power supply with 6CG3s. I have used it in a power supply which had to deliver such a current. That is the same you can get from the big mercury vapour 866A.

The 6CG3 has a 12 pin compactron base. Which is rather unusual in audio amplifiers. This base was developed by General Electric. The idea behind it was to be able to put all kinds of combinations of tube systems into one envelope. There are tubes with this base with 3 triodes in one bottle. This was an attempt to be able to produce for example TV sets with only one socket type. Therefore they also had to put TV dampers on this base. I like to use the 6CG3 in amps which use other compactron base tubes like the 6HS5 or 6HV5A. I also used it in my 211 amp with 6HS5 driver which I already presented in a series of articles. In those amps I use two of the 6CG3 in a voltage doubler arrangement. This rectifier scheme is very demanding on the diodes since rather high peak currents are drawn. The 6CG3 is just right for this and serves reliably in these amps.

The peak inverse voltage is a remarkable 5kV! Although this should be derated for mains rectifier purposes, still it is more than sufficient for just about any audio amplifer. A minor drawback is the higher heater current demand of 1,8A. But still reasonable considering the current capability. Especially if you compare that with the 3A filament durrent of a 5U4. As all TV dampers the 6CG3 has a fairly low voltage drop even at high currents. Although it cannot compete with silicon diodes and not even with the mercury vapour types, it is still remarkable. But it has the benefit of beeing a very low noise rectifier which causes minimal disturbance during switching. The heavy indirectly heated cathode has a nice slow warm up which provides a delay and slow rise of the high voltage. A detailed datasheet of the 6CG3 is available here.

You will notice that often the 6CG3 is labelled with several designations. The ones from General Electric on the photo above are labelled 6CG3/6DE3/6CD3. Many different TV dampers were brought out by the tube manufaturers. It seemed that every one wanted to have his own designation. Most of them were almost the same. At the end of the tube era it was common to label tubes with several designations to cover different types with one article.

The series of photos on the left shows some details of the construction of the 6CG3. The rather thick white spiral which extends out of the cathode sleeve at the top and bottom provides insulation between heater and cathode. This ensures the high heater to cathode voltage which the tube can withstand. The cathode can be up to 5kV peak above the heater potential. The DC voltage difference can still be 900VDC! This means that the heaters of power supplies for voltages up to that level can be referenced to ground. Even in a full wave bridge all 4 diodes can share the same heater winding which is connected to ground. For supplies which need to deliver higher voltages measures need to be taken to keep the potential difference between heater and cathode within the specified limits. Note that the allowed voltage is much smaller with the heater negative to ground. This voltage limit must be maintained otherwise there is the risk of a heater to cathode short. The third photo shows the base with the glass stem in the middle through which the envelope got evacuated during manufacture. Also note the nice copper plate structure on the photo below.

Although compactron tubes are not in wide spread use among tube amp manufacturers or DIYers today, sockets are readily available. The photo below shows old stock plastic sockets and currently manufactured ceramics:

So if you want to use a compactron tube, don't worry about the uncommon socket. It is available through several sources. Should you have trouble finding some, just ask me.

The chinese made ceramic sockets are actually quite nice. Still I prefer the plastic ones. Not because I prefer old stock or dislike chinese sockets. I actually use chinese sockets quite a lot, most of them are ok. What I don't use is chinese tubes. But back to the sockets, what is the problem with the ceramic one? So far I used the 6CG3 in power supplies which deliver voltages in the range of 1200-1300V. The ceramic sockets are intended to be mounted onto the chassis with the metal ring. This leaves quite short creepage and clearance distances between metal parts and socket pins which carry high voltage.

The plastic socket provides larger distances: I also usually do not mount it directly to the metal chassis, but use some stand offs for recessed mounting. In addition I usually drill out the unused pins besides the high voltage carrying ones to enlarge distances. See the photos on the left for details how I did this on a plastic socket.

Pins 3, 5, 9 and 11 are removed. The anode is brought out to pins 4 and 10, so it is just the pins beside the anode connections, which carries the AC input voltage in a full wave rectifier. Pin 7 is connected to the cathode which of cause also carries high voltage (rectified output). The two pins beside the cathode could also be removed. I left them in so that the socket still has a good grip on the tube. With too many removed, the tube would sit quite loose in the socket. This practice enables save operation of the tube even when used with high voltages. Remember high voltages do not need direct contact. Electrons become quite sneaky at voltages in excess of 1kV. They can jump distances through the air, so that a few mm of air is nothing but a short for them.

Ok, the sockets are covered. What about the tube itself? Also no problem to obtain. Almost every major tube dealer has 6CG3s in stock. Often at ridiculously low prices. Sometimes they can be found for a buck or two. I usually buy them in bulk of 100 pcs: